Vinylidene fluoride polymer and method of making same

ABSTRACT

This invention provides PVDF having such crystallization properties as to produce small spherulite sizes without sacrifices to properties, processability, purity, etc., and a suspension polymerization method to obtain the same. 
     The PVDF obtained by this invention comprises the monomer unit content of 99.5-96 wt. % of vinylidene fluoride, and the monomer unit content of 0.5-4 wt. % of comonomers selected from hexafluoropropylene and/or tetrafluoroethylene, and has a logarithmic viscosity of 0.9-1.3 dl/g, and a molecular-weight distribution, as expressed in the ratio (Mw/Mn) of weight-average molecular weight and number-average molecular weight, of 2.2-2.8. 
     In order to manufacture the PVDF of this invention, a mixture of vinylidene fluoride monomers containing 1-5 wt. % of monomers selected from hexafluoropropylene and/or tetrafluoroethylene is suspension-polymerized in an aqueous medium, using an oil-soluble initiator so that the logarithmic viscosity [η 1  ] is kept at 1.3-1.9 dl/g until the polymerization conversion rate of vinylidene fluoride monomers becomes 10-50%, and the polymerization is continued by adding a chain transfer agent at that polymerization conversion rate so that the logarithmic viscosity [η 2  ] of the eventually obtained polymers becomes 0.3-0.7 dl/g lower than [η 1  ], and yet remains in the range of 0.9-1.3 dl/g.

This is a division of application Ser. No. 07/923,487 filed Aug. 3,1992, now U.S. Pat. No. 5,283,302.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates generally to a vinylidene fluoride polymer and amethod of making the same, and more particularly to a vinylidenefluoride polymer (hereinafter referred to as "PVDF" in short) and amethod of manufacturing the same with a suspension polymerizationprocess.

2. Description of the Prior Art

PVDF is now being widely used as a material for piping and valves usedin chemical plants, the lining and coating materials of storage tanksand reaction vessels due to its excellent mechanical strength andchemical resistance. PVDF has such good melt-processability and thermalstability that it can be processed without recourse to thermalstabilizers, processing aids, etc. Due to this unique features, PVDFmoldings have a feature of high purity. Its high purity, together withits excellent chemical resistance, often makes PVDF particularlysuitable for materials for facilities for the manufacture and storage ofultra-pure water and ultra-pure chemicals used in semiconductormanufacture.

PVDF is commercially manufactured by the suspension polymerization andemulsion polymerization processes. The suspension polymerization processis such that monomers are dispersed in droplets in the water as asuspension medium using a dispersant, and are polymerized using organicperoxides dissolved in the monomers to obtain granular polymers of sizesfrom 100 to 300 microns. Suspension polymerization products aremanufactured with a simpler process, and easier to handle because oftheir granular properties, compared with emulsion polymerizationproducts. In addition, suspension polymerization products are of higherpurity than emulsion polymerization products because they do not containemulsifiers or salting agents. Formation of large spherulites inmoldings of suspension polymerization products, on the other hand, maybe regarded as a drawback in some applications.

PVDF is a crystalline resin which tends to form spherulites duringcooling and solidification after melt processing, and it is generallyknown that the surface smoothness of PVDF moldings largely depends onthe size of spherulites.

When PVDF having lowered surface smoothness due to the presence of largespherulites is used to manufacture piping for ultra-pure watermanufacturing lines, minute dents between the spherulites on the insidesurface of the PVDF piping act as a source for propagation ofmicroorganisms, lowering the purity of pure water. When used as coatingsfor reaction vessels and storage tanks, the PVDF paint films havinglarge spherulites are susceptible to stress cracking, reducing theirdurability. It is generally believed that the size of spherulitesdepends on the cooling rate of PVDF moldings after melt processing; thefaster PVDF moldings are cooled, the finer spherulites are formed. Insome molding methods, quenching may be impossible. During the extrusionof thick-wall pipe, for example, when the extruded pipe is cooled fromthe external surface, large spherulites are formed because the inside ofthe pipe is not cooled so quickly, deteriorating the smoothness of thepipe inside. To overcome this problem, therefore, polymers having suchcrystallization characteristics as to form fine spherulites easily evenat a relatively low cooling rate have long been needed.

Means for reducing the size of PVDF spherulites include;

(1) The size of spherulites of PVDF moldings can be reduced to someextent by increasing the molecular weight of polymers. With this method,however, satisfactory effects cannot necessarily be accomplished.Conversely, as the molecular weight is increased, some undesirablephenomena, such as the difficulty in processing due to increased meltviscosity, discoloring caused by the decomposition of polymers due toincreased processing temperature, and formation of hydrogen fluoride dueto increased processing temperature, are encountered.

(2) As proposed in U.S. Pat. No. 3,701,749 and U.S. Pat. No. 3,719,644,the spherulites of PVDF moldings can be reduced in size by adding anucleating agent, such as flavanthrone and salt. Addition of thesenucleating agents is not favorable because it could lower the thermalstability of PVDF, and lead to discoloring caused by the decompositionof polymers, as well as to the formation of hydrogen fluoride.Furthermore, these nucleating agents are impurities in polymers, makingthe polymers unsuitable for applications requiring high purity, such asultra-pure water piping.

(3) It is known that the size of spherulites can be reduced bycopolymerizing vinylidene fluoride monomers with tetrafluoroethylene,trifluoromonochloroethylene, hexafluoropropylene, vinyl fluoride, etc.To sufficiently reduce the size of spherulites with this method,however, more than 10 wt. % of comonomers are required. As a result, thedegree of crystallization of polymers is lowered, and the crystallinemelting point is also lowered remarkably. This leads to deterioration inheat resistance, chemical resistance and mechanical strength asbeneficial properties of PVDF.

(4) U.S. Pat. No. 3,798,287 has proposed a method of adding fluorinemonomers which give the polymer having higher crystallizationtemperatures than PVDF after the polymerization of vinylidene fluoridemonomers has been completed so as to facilitate polymerization withinPVDF particles. With this process, however, vinyl fluoride severelyreduces the thermal stability of the resulting PVDF. Moreover,post-addition of trifluoromonochloroethylene has less effects ofreducing the size of PVDF spherulites, and also lowers the thermalstability of PVDF.

Post-addition of tetrafluoroethylene, on the other hand, remarkablyreduce the size of spherulites and improve the thermal stability ofPVDF, but polytetrafluoroethylene hardly disperse uniformly in PVDF,making the size of the spherulites of PVDF moldings uneven, and posingsome problems in the surface smoothness of moldings.

OBJECT OF THE INVENTION

It is an object of this invention to provide a vinylidene fluoridepolymer (PVDF) having such crystallization characteristics as to formfine spherulite sizes without sacrifices of physical properties,processability, purity, etc., and also to provide a suspensionpolymerization method to obtain the same.

SUMMARY OF THE INVENTION

After years of research efforts to achieve these objectives, the presentinventors discovered that the size of the resulting PVDF spherulites canbe reduced materially by copolymerizing small amounts of comonomers, andyet controlling the molecular weight distribution to preferable valuesduring the suspension polymerization of vinylidene fluoride monomers,and have finally completed this invention.

That is, the PVDF provided by this invention comprises the monomer unitcontent of 99.5-96 wt. % of vinylidene fluoride, and the monomer unitcontent of 0.5-4 wt. % of monomers (hereinafter sometimes referred to as"comonomer") selected from hexafluoropropylene and/ortetrafluoroethylene, with logarithmic viscosities of 0.9-1.3 dl/g, andthe molecular-weight distribution, as expressed by the ratio (Mw/Mn) ofweight-average molecular weight and number-average molecular weight, of2.2-2.8. According to this invention, PVDF can be manufactured by using1-5 wt. % of monomers selected from hexafluoropropylene and/ortetrafluoroethylene as comonomers in the course of the suspensionpolymerization of vinylidene fluoride monomers in an aqueous medium inthe presence of an oil-soluble polymerization initiator (hereinafterreferred to as "polymerization initiator"), continuing thepolymerization so that the logarithmic viscosity [η₁ ] of the polymerbecomes 1.3-1.9 dl/g from the start of polymerization until the rate ofpolymerization conversion (hereinafter referred to as "polymerizationconversion rate") of vinylidene fluoride monomers reaches 10-50%, andfurther continuing polymerization by adding a chain transfer agent whenpolymerization conversion rate reaches 10-50% so that the logarithmicviscosity [η₂ ] of the eventually obtained polymer becomes 0.3-0.7 dl/glower than [η₁ ], and remains at 0.9-1.3 dl/g.

According to this invention, the PVDF having such fine spherulites thathitherto cannot be obtained with the prior art can be easily obtained inthe suspension polymerization process, and as a result, PVDF moldingshaving excellent surface smoothness can be obtained without sacrificesof physical properties, processability and purity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The amount of addition of hexafluoropropylene and/or tetrafluoroethyleneused in this invention should be 1-5 wt. %, preferably 2-4 wt. %, of thetotal monomers. By using them by more than 1 wt. %, the spherulites inthe PVDF moldings can be reduced to a sufficient degree, and by usingless than 5 wt. %, the resulting PVDF is of small spherulites, and thecrystalline melting point and crystallinity of the PVDF do not fallappreciably, and PVDF moldings are excellent in heat resistance,mechanical strength and resistance to organic solvents. Thesehexafluoropropylene and/or tetrafluoroethylene may be added en bloc to apolymerization reactor at the time of initial charging, or a part ortotal amount of them may be added in separate doses or continuouslyduring polymerization.

In this invention, polymerization is effected so that the logarithmicviscosity [η₁ ] of polymers becomes 1.3-1.9 dl/g from the start ofpolymerization till polymerization conversion rate reaches 10-50%,preferably 20-45%. The term logarithmic viscosity used here is acharacteristic value correlated with molecular weight obtained by ameasuring method, which will be described later.

If [η₁ ] is less than 1.3 dl/g, the size of spherulites in the resultingPVDF moldings does not become sufficiently small, while with [η₁ ]exceeding 1.9 dl/g, unmelted PVDF remains at the time of melt molding,resulting in fish eyes, deteriorating the external appearance andsurface smoothness of the moldings.

The timing suitable for reducing logarithmic viscosity by adding a chaintransfer agent in the course of polymerization is the time at which thepolymerization conversion rate reaches 10-50%. If the chain transferagent is added at a polymerization conversion rate of less than 10%, thespherulites of the eventual PVDF do no become sufficiently small, whileat a polymerization conversion rate of over 50%, a large amount of chaintransfer agent is required to limit [η₂ ] in the range of 0.3-0.7 dl/glower than [η₁ ], and to a range of 0.9-1.3 dl/g, and yet the size ofspherulites does not necessarily become small.

The timing for adding the chain transfer agent in the manufacturingmethod of this invention, that is, the time at which the polymerizationconversion rate becomes 10-50% and [η₁ ] becomes 1.3-1.9 dl/g can bedetermined by the change in pressure or polymerizing time at which apredetermined polymerization conversion rate is reached in the followingmanner; by obtaining in advance through preliminary experiments thechange in pressure and polymerization conversion rate when a relativelylarge amount of hexafluoropropylene is charged at the initial stage ofpolymerization, for example; or by obtaining polymerization time andpolymerization conversion rate for such polymerization involving lesspressure changes.

In this invention, the logarithmic viscosity [η₂ ] of the eventuallyobtained PVDF must be kept at 0.9-1.3 dl/g. If this value is less than0.9 dl/g, the size of spherulites in moldings does not becomesufficiently small, resulting in poor surface smoothness of PVDFmoldings, and lowered impact strength. In some cases, this tends tocause stress cracks. If this value exceeds 1.3 dl/g, the melt viscosityof PVDF increases, making melt molding difficult.

Furthermore, [η₂ ] should preferably be lower than [η₁ ] within therange of 0.3-0.7 dl/g, preferably within the range of 0.4-0.5 dl/g. Thedifference between [η₂ ] and [η₁ ] is related to the molecular-weightdistribution of the resulting PVDF. Molecular-weight distribution can beexpressed by the ratio (Mw/Mn) of weight-average molecular weight (Mw)and number-average molecular weight (Mn). If [η₂ ] becomes over 0.7 dl/gsmaller than [η₁ ], the molecular-weight distribution of the resultingPVDF exceeds 2.8. This results in lowered impact resistance and the lossof melting uniformness, adversely affecting the processability of PVDFand the performances of moldings, such as increased fish eyes inmoldings.

In addition, if the difference between both is less than 0.3 dl/g, themolecular weight distribution of the resulting PVDF becomes less than2.2, the effect of sufficiently reducing the size of spherulites cannotbe accomplished.

The logarithmic viscosity of PVDF is determined by polymerizationtemperature, the type and amount of polymerization initiator, the typeand amount of chain transfer agent. That is, with the same types ofpolymerization initiator and chain transfer agent (polymerization aid)used, increased polymerization temperature lowers logarithmic viscosity,while lowered polymerization temperature increases logarithmicviscosity. With polymerization conditions kept constant, except forpolymerization initiator or chain transfer agent, increasing the amountsof these polymerization aids lowers logarithmic viscosity, whereasdecreasing the amounts of these polymerization aids increaseslogarithmic viscosity. By obtaining in advance through preliminaryexperiments the correlationship among polymerization temperature, theformulations of polymerization initiator and chain transfer agent, andlogarithmic viscosity, aimed logarithmic viscosity can be set.Furthermore, the amount of addition of chain transfer agent atpolymerization conversion rate of 10-50% can also be determined in thesame manner. By determining the types of polymerization initiator andchain transfer agent in this way, the aimed logarithmic viscosity can beeasily determined by adjusting the dose.

Chain transfer agents usable in this invention include acetone,isopropyl acetate, ethyl acetate, diethyl carbonate, dimethyl carbonate,pyroethyl carbonate, propionic acid, trifluoroacetate, trifluoroethylalcohol, formaldehyde dimethylacetal, 1,3-butadiene epoxide,1,4-dioxane, β-butyllactone, ethylene carbonate, vinylene carbonate,etc. When taking into consideration the capability of effectivelylowering logarithmic viscosity, the maintenance of the thermal stabilityof PVDF, the ease of availability, and the ease of handling, acetone andethyl acetate, particularly ethyl acetate, are more desirable.

Polymerization initiators usable in this invention include di-n-propylperoxydicarbonate (NPP), and diisopropyl peroxydicarbonate. The typesand amounts of these polymerization initiators and chain transfer agentsare selected so as to obtain a predetermined logarithmic viscosity, andone or more than two types of them can be used in combination.

Typical dispersants usable in this invention include partiallysaponified polyvinylacetate, water-soluble cellulose ethers such asmethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose,hydroxypropylmethyl cellulose, water-soluble polymers such aspolyacrylate, gelatin. The ratio of water/monomers is usually 1.5/1-3/1in terms of weight ratio, and 0.01-0.1 parts by weight of dispersant areused for 100 parts by weight of monomers. In addition, this inventioncan use PH buffers, such as polyphosphates.

As the method of charging water, monomers, dispersant, polymerizationinitiator, and other polymerization aids in this invention, any methodsthat can be used for normal suspension polymerization may be employed.

For example, as water, dispersant, polymerization initiator, chaintransfer agent and other polymerization aids are charged, thendeaeration is carried out by decompression, monomers are subsequentlycharged, and stirring is started. After heating to a predeterminedtemperature, polymerization is started at that temperature, and thechain transfer agent is injected when the polymerization conversion ratereaches 10-50%, and the polymerization is further continued. As thepolymerization proceeds to such an extent that the pressure in thepolymerization reactor drops by more than 10 kg/cm² from the equilibriumpressure of the initial monomer mixture (that is, to the extent that thepolymerization conversion rate reaches over 80%), unreacted monomers arerecovered, and the resulting polymers are dewatered, flushed with waterand dried.

The PVDF obtained with the above manufacturing method consists of themonomer unit content of vinylidene fluoride of 99.5-96 wt. %, preferably99.0-97.0 wt. %, and the monomer unit content of comonomers of 0.5-4 wt.%, preferably 1.0-3.0 wt. %. Combination of both monomer units arerandom. The logarithmic viscosity [η₂ ] is 0.9-1.3 dl/g, as notedearlier, the molecular-weight distribution is 2.2-2.8, preferably2.3-2.6. The size of PVDF spherulites, as measured by the method whichwill be described later, is 1-30 μm, preferably 5-25 μm, and thecrystalline melting point is in the range of 163°-176° C., preferably168°-173° C.

In the following, this invention will be described more specifically,referring to examples and comparative examples. This invention is notlimited to these examples. It should be noted that percentage and partnumbers used in these examples and comparative examples are expressed interms of weight, unless otherwise specified. The property values of PVDFshown in the examples and comparative examples were measured by thefollowing methods.

(1) Molecular-weight distribution (Mw/Mn)

A dimethylacetamide solution in which polymer powders were dissolved ata concentration of 0.2 wt. % was subjected to a gel-permeationchromatograph (manufactured by Tosoh Corp.; 8010 Series, Column TSK-GELGMIIXL, dia. 7.8 mm, length 300 mm, 2 columns in series; temperature 40°C., flow rate 0.8 ml/min.) to measure the ratio (Mw/Mn) ofweight-average molecular weight (Mw) and number-average molecular weight(Mn).

(2) Composition analysis of polymers

Measurements were made using ¹⁹ F NMR.

(3) Crystalline melting point

The endothermic peak was measured with a differential scanningcalorimeter when PVDF was heated at a rate of 10° C./min, and thetemperature at that time was obtained as the crystalline melting point.

(4) Logarithmic viscosity

The logarithmic viscosity was calculated using the following equation onthe basis of the dropping time of a 30° C. solution obtained bydissolving polymer powders into dimethylformamide at a concentration of4 g/l on an Ubbelohde's viscometer.

Logarithmic viscosity [η]=1n(η_(rel))/C dl/g where η_(rel) =droppingtime (sec) of sample solution/dropping time (sec) of solvent

C=Concentration of sample solution (0.4 g/dl).

(5) Spherulite size

Polymer powders were mixed for three minutes in a roll having a rollsurface temperature of 165° C., then 0.1 g of the mixture was pressed byapplying a pressure of 100 kg/cm² at a temperature of 240° C. for threeminutes. The pressed mixture, while kept in the pressurized state, wascooled at a constant rate to 100° C. in 1 hour. The spherulites in theresulting approx. 20 μm-thick film were photographed with a polarizedmicroscope to measure the average value of the spherulite size.

(6) Yield-point strength

As in the case of (5) above, a 1 mm-thick pressed sheet were prepared,and Type-3 dumbbell specimens was taken from this sheet and subjected tomeasurement of a yield point strength at a tension rate of 10 mm/min.and a temperature of 23° C. according to ASTM-D638.

(7) Izod impact strength

The roll-mixed sheet obtained in (5) above was pressed for six minutesat a temperature of 240° C. and a pressure of 100 kg/cm² to obtain a 6mm-thick pressed sheet. In accordance with ASTM-D256, the impactstrength of V-notched (R=0.25 mm) specimens was measured.

(8) Melt viscosity

The roll-mixed sheet obtained in (5) above was cut, and the meltviscosity of the sheet at a temperature of 240° C. and a shear rate of50 sec⁻¹ was measured using a capillograph (manufactured by Toyo SeikiCo., Ltd.).

(9) Fish eyes

The roll-mixed sheet obtained in (5) above was pressure-formed into a 1mm-thick sheet at a temperature of 240° C. and a pressure of 100 kg/cm².This sheet is placed between an iron plate having an air-blow hole, andan iron plate having a 12 cm-dia. circular opening via a rubber packing,and held in place with a clamp. While heating the sheet on the openingside to a surface temperature of approximately 150° C., the sheet isinflated to a thickness of approximately 300 μm by injecting the airfrom the air-blow hole. Then, the number of unmelted material containedin a 5 cm×5 cm area of the resulting film was counted.

(10) Smoothness of pipe inside

In accordance with JIS B0601, the lengthwise smoothness of the inside ofa pipe was measured with a surface roughness tester (manufactured byTokyo Seimitsu Co., Ltd.: Surfcom 550A Type) to obtain the averageroughness Ra for a reference length of 2.5 mm.

EXAMPLE 1

The following monomers and polymerization aids, etc. were charged in astainless steel-made autoclave having an inner capacity of 14 liters,and polymerization reaction was started at 25° C.

    ______________________________________                                        Vinylidene fluoride                                                                              97.5   parts (3,000 g)                                     Hexafluoropropylene                                                                              2.5    parts                                               Pure water         300.0  parts                                               Methyl cellulose   0.1    parts                                               Sodium pyrophosphate                                                                             0.2    parts                                               NPP                0.61   parts                                               ______________________________________                                    

After 3 hours from the start of polymerization (polymerizationconversion rate: 35%), 3.0 parts of ethyl acetate was added and thepolymerization reaction was continued. At a point of time when theinside pressure of polymerization reactor drops 25 kg/cm² below from theequilibrium pressure (39 kg/cm²) after the start of polymerization,unreacted monomers were recovered, and the resulting polymer slurry wasdewatered, flushed with water and dried. The chemical and physicalproperty values of the resulting polymer are shown in Table 1.

EXAMPLE 2-5

The polymerization was conducted in the same manner as in Example 1except that the amounts of NPP and ethyl acetate and the timing of theiradditions, [η₁ ] of the polymer at the addition of ethyl acetate, [η₂ ]of the polymer obtained eventually were changed as shown in Table 1. Thechemical and physical property values of the resulting polymer are shownin Table 1.

EXAMPLE 6

The polymerization was conducted in the same manner as in Example 1,except that the amount of hexafluoropropylene was changed to 4%. Thechemical and physical property values are shown in Table 1.

EXAMPLE 7

The polymerization was conducted in the same manner as in Example 1,except that the amounts of NPP and ethyl acetate, and polymerizationtemperature were changed as shown in Table 1. The chemical and physicalproperty values of the resulting polymer are shown in Table 1.

EXAMPLE 8

The polymerization was conducted in the same manner as in Example 1,except that hexafluoropropylene was replaced with tetrafluoroethylene.The chemical and physical property values are shown in Table 1.

COMPARATIVE EXAMPLES 1-5

The polymerization was conducted in the same manner as in Example 1,except that the amounts of NPP and ethyl acetate, and the timing oftheir additions, [η₁ ] of the polymer at the addition of ethyl acetate,and [η₂ ] of the polymer obtained eventually were changed as shown inTable 2. The chemical and physical property values of the resultingpolymer are shown in Table 2.

COMPARATIVE EXAMPLE 6

The polymerization was conducted in the same manner as in Example 1,except that the amount of hexafluoropropylene was changed to 6%. Thechemical and physical property values of the resulting polymer are shownin Table 2.

COMPARATIVE EXAMPLE 7

The polymerization was conducted in the same manner as in Example 1,except that the amount of NPP was increased and ethyl acetate was notadded in the course of reaction. The chemical and physical propertyvalues are shown in Table 2.

COMPARATIVE EXAMPLE 8

The polymerization was conducted in the same manner as in Example 1except that hexafluoropropylene was not used. The chemical and physicalproperty values of the resulting polymer are shown in Table 2.

COMPARATIVE EXAMPLE 9

The polymerization was conducted in the same manner as in ComparativeExample 7, except that hexafluoropropylene was not used. The chemicaland physical property values of the resulting polymer are shown in Table2.

EXAMPLE 9, COMPARATIVE EXAMPLES 10 AND 11

The polymerization was conducted in a stainless steel-made autoclavehaving an inner capacity of 6,300 liter with the same polymerizationformulations as with Example 1, Comparative Examples 8 and 9, in which1,200 kg of vinylidene fluoride was used (Example 9, ComparativeExamples 10 and 11). The resulting polymer was molded into a pipe havingan outside diameter of 100 mm and a thickness of 6 mm using a 90 mm-dia.single-screw extruder having a cylinder temperature of 180° C. under thehopper and 240° C. at the tip thereof, and a die temperature of 230° C.through a vacuum sizing die equipped with a water-cooled jacket. Theaverage surface roughness Ra of the inside of the resulting pipe was asgood as 0.3 μm (Example 9) with the polymer produced from theformulation of Example 1, compared with 0.9 μm (Comparative Example 10)with the polymer produced from the formulation of Comparative Example 8,and 2.2 μm (Comparative Example 11) with the polymer produced from theformulation of Comparative Example 9.

This invention can be applied in other various ways without significantdeparture from the purpose and main characteristics thereof.Consequently, the aforementioned embodiments represent mere examples inall respects, and should not be construed as restrictive. The scope ofthis invention is defined by the scope of claims, and is by no meansbound by the text of the Specification. Furthermore, all modificationsand alterations belonging to the equivalent scope of the claims fallwithin the scope of this invention.

                                      TABLE 1                                     __________________________________________________________________________                    Example                                                                       1   2   3   4   5   6   7   8                                 __________________________________________________________________________    Comonomer                                                                     Type            HFP HFP HFP HFP HFP HFP HFP TFE                               Charge (parts)  2.5 2.5 2.5 2.5 2.5 4.0 2.5 2.5                               Polymerization initiator                                                      Type            NPP NPP NPP NPP NPP NPP NPP NPP                               Charge (parts)  0.61                                                                              0.61                                                                              0.61                                                                              0.49                                                                              0.75                                                                              0.61                                                                              0.20                                                                              0.61                              Chain transfer agent                                                          Type            EAC EAC EAC EAC EAC EAC EAC EAC                               Addition amount 3.0 6.5 1.2 1.4 4.7 3.0 2.0 3.0                               Polymerization conversion                                                                     35  45  20  35  35  35  35  35                                rate at addition (%)                                                          Logarithmic viscosity [η.sub.1 ]                                                          1.6 1.6 1.6 1.8 1.4 1.58                                                                              1.6 1.62                              at addition (dl/g)                                                            Polymerization temperature                                                                    25  25  25  25  25  25  40  25                                (°C.)                                                                  Polymerization yield                                                                          90  90  90  90  90  90  90  90                                (%)                                                                           Chemical and physical property                                                values of PVDF                                                                Logarithmic viscosity [η.sub.2 ]                                                          1.12                                                                              1.13                                                                              1.16                                                                              1.30                                                                              0.97                                                                              1.10                                                                              1.15                                                                              1.14                              (dl/g)                                                                        [η.sub.1 ]-[η.sub.2 ] (dl/g)                                                          0.48                                                                              0.47                                                                              0.44                                                                              0.50                                                                              0.43                                                                              0.48                                                                              0.45                                                                              0.48                              Monomer unit content of                                                                       1.4 1.4 1.4 1.3 1.5 2.3 1.4 2.8                               comonomers (wt. %)                                                            Molecular-weight distribution                                                                 2.5 2.5 2.3 2.3 2.5 2.6 2.4 2.4                               (Mw/Mn)                                                                       Melt viscosity (× 10.sup.-3 poise)                                                      34  34  35  51  22  34  34  34                                Crystalline melting point (°C.)                                                        172 171 172 171 172 168 170 173                               Spherulite size (μm)                                                                       15  20  20  15  25  15  15  10                                Yield strength (kg/mm.sup.2)                                                                  5.3 5.3 5.3 5.2 5.4 4.9 5.0 5.4                               Izod impact strength                                                                          15  12  18  20  10  23  15  17                                (kg-cm/cm)                                                                    No. of fish eyes                                                                              3   6   6   10  8   3   5   8                                 __________________________________________________________________________     (Note)                                                                        HFP: hexafluoropropylene,                                                     TFE: tetrafluroethylene,                                                      NPP: din-propylperoxydicarbonate,                                             EAC: ethyl acetate                                                       

                                      TABLE 2                                     __________________________________________________________________________                    Comparative Example                                                           1     2   3     4   5   6   7   8   9                         __________________________________________________________________________    Comonomer                                                                     Type            HFP   HFP HFP   HFP HFP HFP HFP --  --                        Charge (parts)  2.5   2.5 2.5   2.5 2.5 6.0 2.5 --  --                        Polymerization initiator                                                      Type            NPP   NPP NPP   NPP NPP NPP NPP NPP NPP                       Charge (parts)  0.37  0.93                                                                              0.61  0.49                                                                              0.61                                                                              0.61                                                                              1.05                                                                              0.61                                                                              1.05                      Chain transfer agent                                                          Type            EAC   EAC EAC   EAC EAC EAC --  EAC --                        Addition amount 10.0  1.3 20.0  0.6 20.0                                                                              3.0 --  3.0 --                        Polymerization conversion                                                                     35    25  25    35  60  35  --  35  --                        rate at addition (%)                                                          Logarithmic viscosity [η.sub.1 ]                                                          2.1   1.2 1.6   1.8 1.6 1.54                                                                              --  1.63                                                                              --                        at addition (dl/g)                                                            Polymerization temperature                                                                    25    25  25    25  25  25  25  25  25                        (°C.)                                                                  Polymerization yield                                                                          90    90  90    90  90  90  90  90  90                        (%)                                                                           Chemical and physical property                                                values of PVDF                                                                Logarithmic viscosity [η.sub.2 ]                                                          1.16  1.0 0.80  1.40                                                                              1.16                                                                              1.07                                                                              1.12                                                                              1.13                                                                              1.14                      (dl/g)                                                                        [η.sub.1 ]-[η.sub.2 ] (dl/g)                                                          0.94  0.20                                                                              0.80  0.40                                                                              0.44                                                                              0.47                                                                              --  0.50                                                                              --                        Monomer unit content of                                                                       1.4   1.2 1.4   1.4 1.4 4.2 1.5 --  --                        comonomers (wt. %)                                                            Molecular-weight distribution                                                                 3.1   2.0 3.5   2.5 2.4 2.6 2.1 2.5 2.1                       (Mw/Mn)                                                                       Melt viscosity (× 10.sup.-3 poise)                                                      35    22  10    60  32  36  34  34  34                        Crystalline melting point (°C.)                                                        172   172 172   172 172 159 172 175 176                       Spherulite size (μm)                                                                       10    100 40    60  50  20  150 80  250                       Yield strength (kg/mm.sup.2)                                                                  5.3   5.4 5.4   5.2 5.3 3.5 5.3 5.6 5.7                       Izod impact strength                                                                          8.5   10  5.5   12  15  20  15  12  15                        (kg-cm/cm)                                                                    No. of fish eyes                                                                              200<  5   200<  30  12  5   4   4   4                         __________________________________________________________________________     (Note)                                                                        HFP: hexafluoropropylene,                                                     TFE: tetrafluroethylene,                                                      NPP: din-propylperoxydicarbonate,                                             EAC: ethyl acetate                                                       

What is claimed is:
 1. A vinylidene fluoride polymer comprising themonomer unit content of 99.5-96 wt. % of vinylidene fluoride and themonomer unit content of 0.5-4 wt. % of monomers selected fromhexafluoropropylene and/or tetrafluoroethylene, and having a logarithmicviscosity of 0.9-1.3 dl/g, a molecular-weight distribution, as expressedby the ratio (Mw/Mn) of weight-average molecular weight andnumber-average molecular weight, 2.2-2.8, a crystalline melting point of163°-176° C. and a spherulite size of 1-30 μm.
 2. A vinylidene fluoridepolymer as set forth in claim 1 comprising the monomer unit content of99.0-97 wt. % of vinylidene fluoride and the monomer unit content of1.0-3 wt. % of monomers selected from hexafluoropropylene and/ortetrafluoroethylene, and having a molecular-weight distribution, asexpressed by the ratio (Mw/Mn) of weight-average molecular weight andnumber-average molecular weight, of 2.3-2.6, a crystalline melting pointof 168°-173° C. and a spherulite size of 5-25 μm.